This invention relates in general to vehicle electric power steering assemblies and in particular to an improved rack drive assembly adapted for use in such a vehicle electric power steering assembly.
One known electric power steering assembly for turning steerable wheels of a vehicle includes a ball nut for transmitting force between an axially movable rack member and an electric motor. Upon actuation of the electric motor of the power steering assembly, the ball nut is driven to rotate relative to the rack member. The rotational force of the ball nut is transmitted to the rack member by balls to drive the rack member axially. The axial movement of the rack member by the balls effects turning movement of the steerable wheels.
The electric motor in the above-described electric power steering assembly can be coupled to drive the ball nut in various known methods. One such method includes a belt drive assembly wherein an output shaft of the electric motor is connected via a belt to a pulley assembly. The pulley assembly typically includes a pulley which is operatively fixed to the ball nut so that the ball nut rotates with the pulley when the electric motor is actuated.
One example of an electric power steering assembly wherein an electric motor output shaft is connected via a belt to a pulley assembly is disclosed in U.S. Pat. No. 4,825,972 to Shimizu. In the Shimuzu patent, an electric motor has a toothed small diameter pulley fixed on an output shaft of the motor. The toothed small diameter pulley is connected through a timing belt to a toothed large diameter pulley formed on an outer circumference of a ball bearing nut. The ball nut is rotatably supported at both ends thereof in a casing by means of angular contact bearings which are axially biased with a plate spring to eliminate axial play. In operation, the rotation of the motor is transmitted to the ball bearing nut which through rolling balls causes a rack shaft to move in a straight linear motion.
This invention relates to an improved structure for a rack drive assembly adapted for use in a vehicle electric power steering assembly. The electric power steering assembly of the present invention includes a steering member, a one piece integral ball nut and pulley member, a lone bearing unit, and an electric motor. The steering member is disposed in a rack housing and includes a rack portion operatively connected to a vehicle steering wheel and a screw portion. The one piece integral ball nut and pulley member is operatively connected to the screw portion of the rack portion of the steering member for effecting axial movement of the steering member upon rotation of the vehicle steering wheel. The ball nut and pulley member includes a ball nut portion and a pulley portion. The lone bearing unit is carried by the ball nut and pulley member for rotatably supporting the ball nut portion relative to the steering member. The electric motor is operatively connected to the ball nut and pulley member by the pulley portion.
Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.